A massacre of up to 80 Yanomami Indians has taken place in the Venezuelan state of Amazonas, according to claims emerging from the region, prompting the government to send in investigators.

Blame is being placed on illegal garimpeiro miners who cross the border from Brazil to prospect for gold and have clashed violently with Amazon tribes before. According to local testimonies an armed group flew over in a helicopter, opening fire with guns and launching explosives into Irotatheri settlement in the High Ocamo area. The village was home to about 80 people and only three had been accounted for as survivors, according to people from a neighbouring village and indigenous rights activists.

The claims were presented to local authorities in Puerto Ayacucho, the capital of Amazonas state on Monday, asking for an immediate investigation of the site where the alleged killing took place, and for the expulsion of the garimpeiros. The event would have taken place during the first two weeks of July but due to the remoteness of the village it is only now been made public.

A spokeswoman at the public prosecutor's office said the government could not yet confirm the attack nor how many people may have been killed.

Luis Shatiwe, a leader of the Yanomami group, told a Venezuelan newspaper that the survivors were hunters who had been out of the village at the time of the alleged attack. The hunters, he said, heard a helicopter and gunfire and said a communal hut in the village was destroyed by fire.

Survival International, a London-based organisation that seeks to protect native peoples, said in a statement that another Yanomami told the group that tribespeople had found bones and charred bodies in the village.

A member of the team that collected the testimony said: "When we heard the first accounts we flew into Parima-B [the closest town] by helicopter with a contingent of military. In Parima we spoke to Yanomami who had walked six days to get to Parima-B to talk to us. In places this remote that is how people communicate." The man asked not to be identified.

Luis Bello, a lawyer in Puerto Ayacucho who defends indigenous rights, said the allegations were the latest in a series of reports of abuse as garimpeiro activities in the region have increased. "Reports of garimpeiros attacking different communities are becoming more and more frequent, and now we also hear of rivers being poisoned with mercury. We've reported to the authorities but we are so far away that is it all easily forgotten," Bello said.

Bello said a combination of high gold prices and pressure from the Brazilian federal police in their own territory had led to the influx of garimpeiros. "They have also become more sophisticated. They used to fly in and land in clandestine strips, now they come in helicopters and use huge extracting machinery that is decimating the jungle," Bello said.

In 1993, 16 Yanomamis were killed by garimpeiros in what became known as the Haximu massacre. But there have been cases that turn out to be fake. Aime Thilet, a member of Wataniba, an NGO that defends indigenous rights, said that when the latest alleged attack was reported "we were in the Alto Siapo, also on the border with Brazil, because we got radio a very detailed and what seemed credible report of another massacre, which turned out to be false".

Livorio Guarulla, the governor of Amazonas state, said remoteness and military restrictions on access to the area made it difficult to investigate the claims quickly. "This happened in July but because it takes close to seven days to get there we don't really know what happened. The shaponos – the collective community dwellings – house more than 100 people, so it could be 70 [casualties] or it could be more or less."